In recent years there has been an increase in the use of microwave communication systems, particularly in dedicated and cellular mobile phone systems. Microwave transmission is very directional and often requires the use of a repeater station to receive a signal transmitted from a first terminal and to redirect it toward a second terminal where it is received.
Various forms of repeaters have been developed to handle this function. These include back-to-back antennas, double passives, regenerative repeaters and RF repeaters. Repeaters typically convert the radio frequency signal to an intermediate frequency or baseband during signal processing. RF repeaters however perform signal amplification directly at radio frequency.
In order to properly maintain a communication system it is important to know the condition of key elements of the system, such as power supply, amplifier operation and site security. Transducers attached to appropriate items in and around a repeater site are used to generate raw monitor signals of the repeater operation. In repeaters which convert the signal to an intermediate or baseband frequency, these monitor signals can be added to the communication signal. In RF repeaters this is not possible because the communication signal is maintained as an RF signal.
The present invention overcomes this inherent inability to apply a monitor or alarm signal to the communication signal at an RF repeater location. It does this by modulating the communication signal with a monitor signal, preferably of low frequency and amplitude.
This feature is provided by a monitoring system comprising means for generating a signal representative of the desired characteristic to be monitored. Also included are means responsive to the generated signal for producing a modulating signal representative of the generated signal and having a frequency substantially less than the communication signal carrier frequency. Means are also provided for modulating the amplitude of the communication signal with the modulating signal, whereby the retransmitted communication signal is representative of the desired characteristic.
The invention also includes a method for performing these functions. In the preferred embodiment, the method includes (a) receiving the communication signal at a repeater unit; (b) generating simultaneously a plurality of signals representative of the status of a corresponding plurality of operating conditions of the repeater unit; (c) time multiplexing the respective status signals to form a combined signal; (d) encoding the combined signal to form a digital signal of frames of data having a frequency substantially less than the communication signal carrier frequency, with a frame being formed of data from each of the status signals; (e) varying the level of the communication signal proportionally with the value of the digital signal; (f) retransmitting the level-varied communication signal from the repeater unit; (g) receiving the retransmitted communication signal at a receiver unit remote from the repeater unit; (h) generating a control signal representative of the signal level of the retransmitted communication signal; (i) amplifying the received retransmitted communication signal inversely proportionally with the value of the control signal for maintaining the value of the amplified retransmitted communication signal at a substantially constant level; (j) regenerating the digital signal from the control signal; and (k) decoding the regenerated digital signal into a plurality of output signals corresponding to and representative of the original operating-condition-status signals. Apparatus for performing three steps are also within the contemplation of the present invention.
It will thus be appreciated that the present invention provides a method and apparatus for monitoring characteristics such as are associated with a repeater unit and modulating the communications signal at the repeater unit with the communication signal still at its transmission frequency to add the monitor information to the signal, and then separate the monitor signal from the communication signal at a receiver terminal. Thus, monitor information is transmitted to a remote terminal for providing status information about the repeater or other information in a manner which is reliable and inexpensive to accomplish.
These and other features and advantages of the present invention will become apparent from a review of the drawings and the following associated detailed description of the preferred embodiment.